This invention generally relates to spread spectrum communication systems. More particularly, the present invention relates to controlling transmission power levels in such systems.
In many spread spectrum communication systems, multiple user communications are sent over a shared frequency spectrum. In code division multiple access (CDMA) communication systems, the multiple communications are sent using different codes to distinguish them. In time division duplex (TDD)/CDMA and time division synchronous CDMA (TD-SCDMA) communication systems, the spectrum is also time divided to better utilize the spectrum.
Since multiple communications are sent in the same spectrum, the communications may interfere with each other. To reduce the interference that the communications induce on one another, adaptive power control is used. The purpose of adaptive power control is to transmit each communication at a minimum power level to achieve a specified reception quality, such as a target signal to interference ratio (SIR).
One scheme to implement adaptive power control is closed loop power control. In closed loop power control, a transmitter transmits a communication at a certain power level. A target receiver receives the communication and measures the SIR of the received signal. The measured SIR is compared to a target SIR. If the measured SIR is larger than the target SIR, a power command indicating that the transmitter should decrease its transmission power is sent. If the measured SIR is less than the target SIR, a power command indicating that the transmitter should increase its power level is sent. The power command is, typically, a single bit indicating either an increase or decrease in power level. The transmitter receives the power command and changes its transmission power level in response to the power command by a fixed amount, such as by increasing or decreasing the transmission power level by 1 dB.
When there is a pause in the transmissions, no data is present in the channel for SIR calculations. In this situation, the receiver may generate either no power commands or dummy power commands. The dummy commands are arbitrary commands and most likely have no relation to any actual change in channel conditions. Likewise, when there is a pause in the receiver's transmissions, no power commands are sent to the transmitter. Since the channel conditions may change during these pauses, the transmission power level may deviate from the desired power level for the current channel conditions. If the power level is too low, a desired quality of service (QOS) may not be achieved. If the power level is too high, unnecessary interference may be introduced onto other users.
Accordingly, it is desirable to have other approaches to power control.